An internal combustion engine is typically provided with an oil pump which pumps lubrication oil under pressure into the bearing clearance between a pair of half bearings and a rotating shaft in the bearing assembly. The lubrication oil is pumped to the inner face of one or both half bearings (e.g. into a groove in the inner face of one or both half bearings). High oil pressure is required to hydrodynamically lubricate the bearing, i.e. maintain the oil film separating the half bearings and the rotating shaft. The inherent nature of bearings typically employed in engines is that they permit axial side oil leakage from within the bearing clearance. Accordingly, engines commonly require oil pumps to have a high flow rate in order to maintain the oil pressure within the bearing clearance.
The bearing assembly has a rotatable shaft held between two halves of a housing (e.g. an engine housing comprising an engine cap and an engine block, or a connecting rod housing). Commonly the housing is assembled with a pair of bearing shells respectively providing running surfaces that face the rotating shaft. Alternatively, the running surface may be provided on the housing (typically with additional coatings provided on the surface of the housing), without the use of bearing shells between the shaft and housing.
Oil leakage is particularly pronounced in regions adjacent the end faces of the half bearing, as commonly, the inner faces of half bearings are provided with relief regions extending from the end faces (joint faces). The relief regions comprise crush relief regions (bore relief regions) and/or eccentric relief regions. In use, the half bearing has a concave inner surface that is generally concentric with the axis of rotation of the shaft within the bearing assembly. The relief regions adjacent the end faces provide a locally increased bearing clearance, which increases towards the adjacent end face.
The crush relief regions are regions of the concave inner face of the bearing of the half bearing in which the bearing clearance is wider, that extends no more than 30° from the adjacent end face, and typically extends no more than 10°. They are used to avoid the danger of a small step in the assembled bore, at the joint between the pair of complementary half bearings. Such a step could otherwise arise due to any of: misalignment between parts of the housing; wall thickness variations between upper and lower bearing shells; or localised swelling or yielding of a bearing shell at the end face under compression (bearing shells are compressed circumferentially in the assembled bearing, to provide an interference fit with the housing). In the case of a generally semi-cylindrical bearing shell, the crush reliefs are typically regions of reduced wall thickness, on the concave inner surface of the half bearing, extending from the end faces of the half bearings.
The eccentric relief regions are longer regions that are machined (e.g. bored) to provide a greater bearing clearance than at the crown (mid-way circumferentially between the end faces). Commonly the eccentric relief regions extend to or close to the crown. For example the eccentric reliefs may be machined to be curved about a centre of curvature that is slightly removed from the corresponding half bearing, relative to the centre of rotation of the shaft, and which has a slightly larger radius of curvature than the separation between the axis of rotation and the internal face at the crown. Eccentricity controls movement of the shaft, in use, to reduce engine noise, whilst providing adequate oil flow to dissipate heat from the bearing. The eccentric relief regions extend no more than 90° from the adjacent end face.
Accordingly, each relief region provides a bearing clearance that is wider (measured along a radius from the axis of rotation of the shaft) than in the crown region. However, to allow for manufacturing tolerances of the housing, the relief regions are designed to provide a greater increase in bearing clearance than is desirable, leading to increased leakage of the lubricating oil from within the bearing clearance, consequently necessitating an oil pump capable of a higher flow rate. Such pumps have several disadvantages including: much energy being wasted by the pump being physically too large and consuming too much engine power to drive it; the oil pump being unnecessarily heavy; and, under some operating conditions (e.g. when starting a cold engine with highly viscous oil, or at high rotational speeds) the pump may provide too much oil pressure and the oil flow may be diverted straight back into the engine sump via an oil pressure relief overflow valve without ever passing through the bearings.